AT & T global network client unable to connect through ISA 2004

When I try to connect to a remote network using AT&T global network client, i get the following errors:

"Timed out waiting for authentication server to respond. Please verify that you have network connection or contact your network administrator for assistance(RIG0403E)."

Then I press OK.
One more Message
"!Error 403 Socket connect call Failed"

I'm using AT & T global network client, connection is going through an ISA 2004 server. When i disable web proxy authentication, it connects but when i enable web proxy authentication, it gives me those errors.

Thanks